1. Field of the Invention
The present invention relates to an automatic document feeder used in an image forming apparatus such as a copying machine.
2. Description of the Prior Art
FIG. 8 shows the construction of a paper feeding section of an automatic document feeder used in a copying machine. The paper feeding section of the automatic document feeder comprises a paper feeding roller 100, a pair of paper separation rollers 101 and 102, and a pair of registration rollers 103 and 104. The paper feeding roller 100, the pair of paper separation rollers 101 and 102, and the pair of registration rollers 103 and 104 are driven by a paper feeding motor (not shown). The pair of paper separation rollers 101 and 102 comprises a forward roller 102 rotated in the direction in which documents are fed and a reverse roller 101 rotated in the opposite direction to the direction in which documents are fed as well known, to prevent the documents from being fed with their being overlapped with each other.
The rotating force of the paper feeding motor is transmitted to the paper feeding roller 100 and the pair of paper separation rollers 101 and 102 through a paper feeding clutch. The rotating force of the paper feeding motor is transmitted to one of the pair of registration rollers 103 and 104 through a registration clutch.
When a print key is activated so that a start signal from a main control section of the copying machine is sent to a control section of the automatic document feeder, the paper feeding motor is driven, so that the paper feeding roller 100 and the pair of paper separation rollers 101 and 102 are driven for a constant time period (primary paper feeding). A document P is usually fed toward the pair of registration rollers 103 and 104 by the primary paper feeding, and the front end of the document abuts on the pair of registration rollers 103 and 104, so that a loop is formed in the front end of the document.
Thereafter, when a document change signal from the main control section of the copying machine is sent to the control section of the automatic document feeder, the paper feeding roller 100, the pair of paper separation rollers 101 and 102, and the pair of registration rollers 103 and 104 are driven for a constant time period (secondary paper feeding). The document P is fed to a delivery belt provided ahead of the pair of registration rollers 103 and 104 in the direction in which documents are conveyed by the secondary paper feeding, to be set in the position for document reading on a transparent platen in the main body of the copying machine by the delivery belt.
When the document is set in the position for document reading, a print signal is sent to the main control section of the copying machine from the control section of the automatic document feeder, to report that a copying operation is possible.
After the primary paper feeding, the front end of the document is usually nipped between the pair of registration rollers 103 and 104. Even after the primary paper feeding, however, the front end of the document may not, in some cases, be nipped between the pair of registration rollers 103 and 104 depending on, for example, the type of print stack used. Thus, the lead edge of the document may not, in some cases, be nipped between the pair of registration rollers 103 and 104 even after the primary paper feeding. In the secondary paper feeding, therefore, not only the pair of registration rollers 103 and 104 but also the pair of paper separation rollers 101 and 102 is driven.
After the primary paper feeding, there occurs the difference in time required to move the document to the position for document reading in the secondary paper feeding depending on a case where the front end of the document is nipped between the pair of registration rollers 103 and 104 or a case where the front end of the document is not nipped between the pair of registration rollers 103 and 104. Specifically, a time period elapsed from the time when the control section of the automatic document feeder accepts the document change signal until it outputs the print signal (a time period required to change documents) varies.
In the conventional automatic document feeder, the time period required to change documents thus varies. Accordingly, an interval at which recording paper is conveyed to the position for transfer must be made relatively longer in view of the variation in the time period required to change documents. Therefore, the variation in the time period required to change documents is a factor which prevents the copy speed from being increased.
An object of the present invention is to provide an automatic document feeder capable of avoiding the variation in a time period required to change documents and increasing the copy speed.